The connections on a PCB should be identical to its corresponding circuit diagram, but while the circuit diagram is arranged to be readable, the PCB layout is arranged to be functional, so there is rarely any visible correlation.

There are three aspects in PCB layout: right angle, differential and snake-like routing.

Right angle

The influence of right angle routing on signal is mainly reflected in three aspects:

A. The corner can be equivalent to the capacitive load on the transmission line, slowing the rise time;

B. Second, the incontinuity of impedance will cause the signal reflection;

C. From EMI produced by the right-angle tip to the RF design field above 10GHz, these small right angles may become key objects of a high-speed problem.

Differential routing

What is a differential signal? In vulgar terms, the drive side sends two equal and inverted signals, and the receiver determines whether the logic state is "0" or "1" by comparing the difference between the two voltages. And the pair of traces that carry the differential signal are called differential traces. Compared to ordinary single-ended signal routing, the most obvious advantages of differential signal are reflected in the following three aspects:

1. Anti-interference ability is strong. The coupling between the two differential traces is very good. When there is noise interference outside, it is almost simultaneously coupled to the two lines, and the receiver is only concerned about the difference between the two signals, so the external common mode noise can be completely offset.

2. Differential signal can effectively suppress EMI. Similarly, because the polarity of two signals is on the contrary, their external radiation electromagnetic field can offset each other, the closer the coupling, the less the electromagnetic energy released to the outside world.

3. Timing positioning is accurate. As the differential signal switching is located in the intersection of two signals, rather than ordinary single-ended signal relies on the level of two threshold voltage to determine, so the impact of process and temperature is small, reducing the timing error, which is more suitable for low-amplitude signal circuit. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technology.

Snake-like routing (adjustment delay)

The snake-like routing is a type of route that is often used in layout. Its main purpose is to adjust the delay to meet the requirements of system timing design. The most two critical parameters are the parallel coupling length (Lp) and the coupling distance (S), apparently, lines parallel to each other will be coupled in differential mode when the signal in the serpentine line transmission. The larger the Lp and S are, the greater the degree of coupling will be. This may lead to reduced transmission delay, and greatly reduce the quality of the signal due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk. The following is the proposal for the layout engineer to deal with the snake-like routing:

1. To increase the parallel line distance (S), at least greater than 3H. H refers to the distance from signal trace to the reference plane. Generally, it means routing around the big bend, as long as S is large enough, mutual coupling effect can almost be completely avoided.

2. Reduce the coupling length (Lp), when the double Lp delay close to or exceed the signal rise time, the resulting crosstalk will reach saturation.

3. The signal transmission delay caused by strip-line or snake-like lines embedded Micro-strip is less than the (Micro-strip). In theory, the strip-line does not affect the transmission rate due to differential mode crosstalk.

4. Try to prevent snake-like routing in the signal lines of stringent high-speed and timing requirements, in particular, a small range of winding route cannot be allowed.

5. Frequently using snake-like routing of any angle can effectively reduce the coupling between each other.

6. In high-speed PCB design, the snake-like routing does not have the so-called filtering or anti-jamming capability, and it only reduces the signal quality for timing matching with no other purpose.

7. Sometimes you can consider the spiral routing, and simulation shows that the effect is better than the normal snake-like routing.